4. Structure/ Composition of Earth

Question 1

What is Mantle convection?

Answer 1

• process that recycles basalt
• created at spreading ridges, passed along oceanic crust until subducted again
• whole- mantle versus layered- mantle

Question 2

What is decompression melting/ Adriatic rise?

Answer 2

• When convection currents bring mantle rock up beneath the spreading ridge

Question 3

What is incongruent melting?

Answer 3

• when a rock doesn’t melt uniformly.

- Partially melting a rock means the melt has a different composition from the bulk rock

Question 4

Talk about incongruent melting in the context of the Earth’s mantle?

Answer 4

• basalt is a partial extract of the mantle, leaving behind peridotite
• peridotite + basalt = rock that basalt is made of
• could be garnet- peridotite / Lherzolite or pyrolite (has peridotite and basalt)

Question 5

What is whole-mantle convection versus layered mantle convection?

Answer 5

• whole mantle: convection currents carrying heat from the interior to the planet’s surface
• layered-mantle- convection currents occur at lower and upper mantle

Question 6

What is the transition zone?

Answer 6

• between upper and lower mantle where seismic waves change velocity because of different composition
• where minerals absorb huge amount of water in crystal structures
• 410-660 km
• high temperature and pressures compress olivine to wadsleyite; deeper down, wadsleyite–> ringwoodite–> bridgmanite and ferropericlase

Question 7

Tell the km of upper mantle, transition zone, lower mantle, Core-mantle boundary.

Answer 7

0-410- upper mantle
410-660 transition zone
660- 2900 lower mantle
2900 core-mantle boundary

Question 8

Who was Richard Oldham? Who was Inge Lehmann?

Answer 8

• Richard Oldman: British seismologist who discovered the core. 1906
• Inge Lehmann: Danish seismologist who discovered liquid outer core

Question 9

What is the geodynamo?

Answer 9

• Earth’s magnetic field

- has directionality- a positive and negative end that may be steered by convection currents.

Question 10

What is the most common rock on Earth’s surface?

Answer 10

Basalt. Because it forms oceanic crust, which covers 75% of Earth’s surface

• generated at spreading ridges
• recycled through mantle convection

Question 11

Do we know if it’s whole-mantle versus layered-mantle convection?

Answer 11

Nope. We don’t know the answer.

Question 12

What is the Earth’s core largely made of? Are there oxides in crust?

Answer 12

• metallic iron and iron-nickel alloy

- no metallic iron in crust because there’s too much oxygen, which turns iron into different oxides

Question 13

What is the lower mantle made of?

Answer 13

Silciate materials

Question 14

How fast do seismic waves travel?

Answer 14

Few thousand meters/ second

7000 miles/ hour

Question 15

How do we know the outer core is liquid?

Answer 15

• We know P-waves travel through liquid, but S-waves don’t because liquid isn’t shear enough
• When an earthquake happens, we see P-waves, but don’t see S-waves on opposite side of earth. Absence of S waves mean outer core is molten

Question 16

Tell me about earth’s magnetic field.

Answer 16

• generated at liquid outer core
• protects Earth from solar wind-supercharged particles from sun that would cause UV radiation, damage ozone.
• direction changes every hundred thousand years. Last was 800,000 yrs ago.
• direction recorded by ocean crust

Question 17

When was the last time the magnetic field changed direction? How do we know?

Answer 17

• 800,000 years ago magnetic field flipped

- the direction was recorded by the ridge’s ocean crust

Question 18

What is the structure of the inner core?

Answer 18

• inner core behaves like giant crystal
• iron is HCP- hexagonal-closed packed
• Iron is anisotropic- meaning seismic waves pass through one direction of crystal faster than the other. Seismic waves pass through inner core faster top down than side by side

Question 19

What structure is iron at inner core versus room temp/pressure?

Answer 19

Hexagonal closed packed at core

Room pressure/ temperature- iron is body- centered cubic

Question 20

What is anisotropic?

Answer 20

Waves pass through one direction faster than the other.

- Inner core of Earth, iron is hexagonal-closed packed. Seismic waves pass faster top down rather than side by side.